1. Field of the Invention
The present invention relates to a motor rotation angle detection device, for detecting, using a resolver, a rotation angle of a motor generating an assist torque for steering road wheels, wherein the resolver is configured by arranging an outer circumferential surface of an annular rotor inside an inner circumferential surface of an annular stator with an air gap interposed in between, so that the stator and the rotor are rotatable relative to each other.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2005-247079 has made publicly-known an electric power steering system for assisting the steering operation of a driver by use of a torque generated by an assist motor which is operated depending on a steering torque which is inputted into a steering wheel by the driver and then is detected by a steering torque sensor.
In a case where a DC brushless motor is employed as the assist motor for the electric power steering system, it is necessary to detect a rotation angle (phase) of a rotary shaft of the assist motor for the purpose of controlling the application of currents to the U-phase, V-phase and W-phase coils of the stator of the motor. A variable reluctance resolver, generally used to detect the rotation angle, includes a stator and a rotor which are opposed to each other. The stator is fixed to a housing of the assist motor, and the rotor is provided on a rotary shaft of the assist motor. The variable reluctance resolver is designed to detect a rotation angle of the rotary shaft based on change in the air gap between the outer circumferential surface of the rotor and the inner circumferential surface of the stator. In this respect, concaves and convexes are formed on the outer circumferential surface of the rotor, and the stator is formed of multiple coils.
However, problems arise in using a resolver such as that disclosed in JP 2005-247079. For example, when the rotor is press-fitted and fixed to the rotary shaft of the assist motor, magnetic permeability becomes lower in some parts of the rotor due to residual stress caused by the press-fit. This lower magnetic permeability leads to reduction in the accuracy of the resolver in detecting the rotation angle. Moreover, when a distortion torque is applied to the rotary shaft of the assist motor, a stress is generated by the distortion torque, thus decreasing the magnetic permeability. This decrease in the magnetic permeability reduces the accuracy of the resolver in detecting the rotation angle.